The present invention relates to fluid couplings, and in particular the couplings which are used in high pressure hydraulic environments, as encountered on machine tools, agricultural and construction equipment, and the like.
For many years, the hydraulics industry has relied on hydraulic couplings for use in manifolds, pump housings, and control units that comply with the S.A.E. J514 standard. These couplings utilize an O-ring, which are known in the industry as O-ring boss fittings. O-ring boss fittings are significantly better than the metal to metal couplings used elsewhere in hydraulic systems, and function well in lower pressure environments. However, current hydraulic applications utilize significant higher pressures, and the application of hydraulics to robotics and other environmentally sensitive applications dictates that all hydraulic couplings be leak free.
A major reason for the failure of certain prior art O-ring fittings is the rotational loosening of threads which occurs under conditions of high pressure and vibration. These conditions cause a gap between the back face of the hydraulic fitting and the front face of the hydraulic port to open up, which eventually leads to the O-ring material being extruded under pressure into the interface. As the fitting is screwed into the port, the O-ring is compressed and becomes, in all respects, another solid component, incapable of responding to pressure differentials within the interface area. Even in the case where the O-ring may not fill the entire space but fills a very high percentage of the space, the O-ring will be incapable of experiencing any significant axial movement and will function similarly to the case where the O-ring substantially fills the entire space.
As hydraulic pressure is exerted on the O-ring, it is urged outwardly and is extruded through a fitting gap. When the hydraulic pressure is subsequently released, the extruded portion of the O-ring is pinched by the abutting faces of the fitting and manifold as they settle back together. This occurs repeatedly upon application and relaxation of pressure, until failure eventually occurs. It is undesirable for the O-ring to be under high compressive forces, because this causes it to function as a packing material that is not capable of significant axial movement within the space.
Other prior art techniques have been attempted in an effort to provide improved hydraulic sealing. For example, the European industry has utilized bonded seals, which comprise a metal washer to which is bonded a rubber washer of special cross-sectional shape, wherein the metal washer provides a backup to the rubber seal. Although such bonded seals work effectively in many applications, they are relatively expensive to manufacture, and the seals are difficult to use, often resulting in poor assembly and subsequent field failure. Another attempted solution is the use of O-ring boss components which are made with additional threads, whereby the additional friction afforded helps to resist rotational loosening of the components. This is an additional cost, and has not proved to be as effective as industry demands. A further type of prior art seal comprises an O-ring having a resilient spring as its core covered with a softer outer layer. This type of O-ring is not readily deformable and is designed to seal by the action of radial forces from the interference fit between the O-ring and the surface between which it is seated.
In prior art fittings, the O-ring seal is typically located between the hydraulic fluid within the system and the threads of the hydraulic coupling, thereby exposing the threads to the ambient atmosphere. This leaves the threads exposed to possible corrosion by electrolytic action and atmospheric attack, thereby making it difficult to disconnect the coupling. Should this corrosion occur at the interface between the hydraulic conveyance system and the manifold to which it attaches, it would be impossible to disassemble the system for repair or maintenance without damaging expensive components, such as the pumps and controls into which the connecting ports are machined. The problem of thread corrosion can be eliminated by locating the threads in communication with the hydraulic fluid. However, a major reason for failure in service of an O-ring boss fitting is the loosening of the threads which occurs under conditions of pressure and vibration, and this situation is worsened by the lubrication of the threads by the hydraulic fluid in the system.
What is needed is a hydraulic coupling having an adjustable O-ring port fitting which allows for greater adjustment during the assembly of a hydraulic coupling.